[Introductory remarks by Mary Jo Nye]

S.S. Schweber: A perusal of my articles in the history of science indicates that a good many of them have been biographical studies. I have written on Charles Darwin, John Herschel, Auguste Comte, John Slater,¹ and my book on the history of quantum electrodynamics contains fairly lengthy biographical accounts of Sin-Itiro Tomonaga, Julian Schwinger, Richard Feynman and Freeman Dyson and shorter sketches of the lives of, among others, Paul Dirac, Pascual Jordan, Willis Lamb, Wolfgang Pauli, Norman Kroll and Quin Luttinger.² From the names that I have mentioned it is obvious that there is a common feature to all my biographical studies. I have been primarily concerned with remarkable individuals, many of them off-scale, in the sense that their capacities lie at the outer limit of whatever distribution represents human capabilities: individuals whose creativity astounds and whose ability to synthesize overwhelms. I have found the biographical study of these individuals rewarding and I have been attracted to the particular resolutions that biographical studies offer to the challenges of writing history: they focus on individuals and their interactions, and they present a ready chronology -- a valuable asset in any historical narration. Scientific biographies also give insights on how to answer important questions, such as: How is the scientific enterprise shaped internally at any given time? How is it affected by forces outside its boundaries? What are the specific mechanisms by which science influences society, politics, and culture? How are borders established between scientific disciplines, between science and applied science, between science and technology, between science and ideology and politics under various conditions? How are the boundaries negotiated by those who want to abolish them and those who wish to move across them back and forth?

Writing biography is of course also an autobiographical exploration, and a brief account of my own career may help clarify the particular foci of my past biographical researches as well as the structure of my present project: writing the biography of Hans Bethe.

I used to be a theoretical physicist. I did my graduate studies at Princeton from 1949 to 1952, and wrote a dissertation in quantum field theory with Arthur Wightman. Princeton was an enormously stimulating place. During my stay, theoretical physics there was dominated by Eugene Wigner, who was the Jones Professor of Mathematical Physics at the university, and by Robert Oppenheimer, who had recently become the director of the Institute for Advanced Study. Every Friday the departmental colloquium would bring Oppenheimer to the University, and the comments by Wigner and Oppenheimer would expose their rivalry and sharply contrasting views of the world. Every Wednesday afternoon all the theorists -- graduate students and faculty -- would trek to the Institute to attend the seminar there and listen to Oppenheimer's usually acerbic comments on the presentation.

In the fall of 1952 I went to Cornell as a postdoctoral fellow to work with Hans Bethe. Freeman Dyson was also there; he had been appointed the previous year to a full professorship to replace Richard Feynman who had gone to Caltech.³ I was one of a large contingent of postdoctoral fellows and research associates who together with the half-dozen or so graduate students in theory formed a lively intellectual community. We all had offices in the recently built Newman Laboratory for Nuclear Studies, and every day we all went out to lunch at the nearby cafeteria run by Cornell's department of home economics. Bethe would join us whenever he was in town. I recall that on arriving at Cornell I was struck by the fact that the offices of Bob Wilson, the director of the laboratory, and that of Bethe were always open. The professors' doors at Princeton had always been closed. The intensity and the closeness of the interaction between theorists and experimenters was also immediately apparent. Bethe and Wilson shared a secretary whose station was at the entrance to their offices.⁴ Every weekend there would be a party at the house of one of the post-doctoral fellows or research associates to which everyone would be invited. It was not unusual for people to get drunk on these occasions -- for no one had any apprehensions about letting their guards down.

A pervasive sense of community is the strongest impression I have of Cornell. At the time it was not clear to me what or who was responsible for the sense of community. I took for granted the various fora that melded the community: in the Nuclear Lab, the weekly Friday afternoon gatherings where theorists and experimenters presented their latest findings in a form that everyone would understand, and the weekly Monday evening journal club meetings where the latest papers and preprints would be discussed critically for the benefit of everyone. In addition there were the departmental colloquia that brought the Newman Lab staff together with the solid state physicists every week on Monday afternoon. In the Nuclear Lab the sense of community among theorists was consolidated by the fact that most of us during the two years I spent at Cornell were engaged in a collective research program on meson-nucleon scattering.

I came to Brandeis in the fall of 1955 and have been there ever since. Although I became deeply involved in the building of this university which was founded in 1948, and in particular with the creation of its department of physics, I managed to keep up with developments in physics and to remain active in research until the mid-60's. Events during the second half of the decade were, however, disruptive. Quantum field theory -- my field of research -- fell into disrepute. The Vietnam conflict and the ensuing student unrest sucked me more deeply into campus politics. When field theory came back into favor in the late 60's, broken symmetry and non Abelian gauge theories were thrust center stage, but, for one reason or another, I had difficulty adjusting to the new viewpoints -- and by the early 70's I found myself unproductive in physics. At that same time, the pressures connected with justifying the large size of the physics faculty led the department to offer courses that would attract non-science majors. I had always been interested in such activities and since I felt that I was no longer a creative theoretical physicist I volunteered to teach a course on the introduction of probability into the sciences. With the characteristic hubris of the physicist I had always thought that physicists had been responsible for the introduction of statistical and probabilistic methods into the sciences -- think of Maxwell and of Boltzmann and the second law of thermodynamics. Darwin and eighteenth century political economy proved me wrong. Reading Howard Gruber's Darwin on Man opened up a whole new world for me and I decided to spend a sabbatical leave during 1976-77 in the department of the history of science at Harvard. I have been a historian of science ever since.

My first activities as a historian concerned the young Charles Darwin and the genesis of natural selection and the young Auguste Comte and the formulation of the nebular hypothesis. If I was not conscious of it at first, it soon became clear to me that my writing about young creative scientists was an attempt to better understand the roots of creativity.⁵ I believed that had I known more about the education, habits, styles of reasoning of physicists like Bethe, Dyson, Feynman, and so forth, I would have been a more productive physicist. I also came to see my concern with off-scale individuals as stemming from the fact that I had been socialized into a culture that attributes the past successes of the discipline as the accomplishments of great individuals, usually men, who had the ability to read nature better than others. In addition, most of my scientific activities were carried out in a field whose research program at any one time was conceived, formulated, and usually dominated by a single individual: Schwinger, Feynman, Dyson, Gell-Mann, Mandelstam, Chew, Weinberg.

I am presently at work writing the biography of Hans Bethe and it may be helpful at this point to give a brief sketch of his life. He was born on 2 July, 1906, in Strasbourg, when Alsace was part of the Wilhelminian empire. He was an only child. His father, Albrecht Bethe, was a widely respected physiologist who accepted a professorship in Frankfurt when Hans was nine years old. His mother was raised in Strasbourg where her father had been a professor of medicine. Bethe grew up in a Christian household, but one in which religion did not play an important role. His father was Protestant; his mother had been Jewish but she became a Lutheran, before she met Hans' father, at the ceremony at which her older sister converted in order to marry a German army officer. Bethe's mother was a talented and accomplished musician, but a year or two before World War I her hearing was impaired as a result of contracting influenza. The illness affected not only her sense of pitch and put an end to her public performances as a singer, but it also left psychological scars. Everyone who knew Bethe's mother remembers her as a very difficult person, who was prone to what was diagnosed at the time as bouts of "nervous exhaustion," extended periods of depression. The marriage suffered under the strain and the parents were eventually divorced in 1927.

Bethe's talents, particularly his numerical and mathematical abilities, manifested themselves early. By the time he finished Gymnasium he knew he wanted to be a physicist and his poor manual dexterity steered him into theory. After completing a year of studies at the University in Frankfurt he went to Arnold Sommerfeld's seminar in Munich. Bethe obtained his doctorate -- summa cum laude -- in 1928. During 1930 and again in 1931 he spent six months in Rome working with Enrico Fermi. By 1933 he was recognized as one of the outstanding theorists of his generation. His book-length Handbuch der Physik articles on the quantum theory of one- and two-electron systems and on the quantum theory of solids became classics as soon as they were published. In April 1933, after Hitler's accession to power, he was removed from his position in Tübigen because he had two Jewish grandparents. He went to England and in the fall of 1934 he accepted a position at Cornell University. He arrived in Ithaca in February 1935 and has been there ever since. During the war he was the head of the theoretical division at Los Alamos. After the war he was deeply involved in the peaceful applications of nuclear power, in investigating the feasibility of developing fusion bombs and ballistic missiles, and later in designing them. He served on numerous advisory committees to the government including the PSAC, the President's Science Advisory Committee. In 1967 he won the Nobel Prize for his researches in 1938 explaining the mechanism of energy production in stars. His most recent researches have been concerned with the life cycle of supernovas and the properties of the neutrinos involved in the fusion process in the sun. He has been and continues to be an enormously productive scientist.

Bethe is much more of an analytical thinker than a visual thinker. His modes of thought are logical, incremental, additive. Indeed, he makes great use of the vast store of information accurately imprinted in his prodigious memory. When tackling a problem, he invariably relies on the numerous models of analogous systems or processes that he has previously analyzed and solved in order to fashion an applicable approach or model. The stored "pieces" yield clues and insights and are reassembled in new ways. Often the reconnected parts yield a synthesis that results in new coherence, and with it novelty and fructification. Bethe has not invented new modes of expression -- he is perfectly content to learn and use the extant modalities. For him, physical theories are simplified, algorithmic representations of the structure of the real world, and mathematics is the language used for their symbolic representations. These representations are constrained by the physical structure of the world. Bethe delights in the constraints and in the inner logic of mathematics that allows new information, new ways of understanding and looking at the world to emerge from the manipulations of its symbols.

There is, I believe, behind Bethe's scientific researches and his writings a consciousness at work that can be discerned in the way he "explores, orders, combines and transposes a set of basic themes." One of the basic themes in Bethe's work is the identification of the "elementary" entities -- the building blocks -- of the various realms of nature that he has investigated and the characterization of the interactions between them in as "fundamental" a way as possible. This, of course, has been the research program of physicists since the beginning of the century. But Bethe is interested in more: he wants to explain how new structures -- new wholes -- can be built out of the elementary entities. He has done so for atoms, molecules and solids, for nuclei, for stars. Initially, it is the explanation of the stability of the composite structures that interests him -- but eventually it is the history, the evolution of such systems that concerns him. The most obvious examples are his researches on stellar evolution and more recently his investigations of the death throes of supernovas.

Unity and stability are recurring foci in his works. The search for unity and stability, and the expression of union and unity have also been constant themes in his personal life.

Bethe grew up in a world that had been sundered and was unstable. World War I and Weimar Germany shaped the political and economic context of his teens. His personal world was similarly in disequilibrium. Bethe as a child was often sick, and he had to be privately tutored. During World War I when he was ten years old he contracted TB. He was diagnosed as having the illness in the summer of 1916 and that fall he was sent to a children's home in Kreutznach where he stayed until February 1918.⁶ He essentially had no close friends until he entered Gymnasium in Frankfurt. His parents did not entertain much and had few visitors; he characterized their home as a "quiet place; a very quiet place." Nor did they have any connection with a religious community that might have given some ballast to their social world.⁷ Periodically, Bethe's mother needed to go to a "rest home." As Bethe grew up, his parents drifted apart and eventually divorced.

Bethe was close to his father, and it was his father who introduced him to the world of science and encouraged him to pursue his interests in mathematics and the sciences. His father's physiological researches involved what Whitehead has described as "the taking of an object out of its natural setting, out of the normally rich context of mediations that reality offers, in order to consider its intrinsic properties in isolation of the world around it."⁸ Coupled to this Cartesian outlook was also a belief that the world could be reconstituted marginally and additively: the calculus is the emblematic paragon of that world view.⁹

The young Bethe responded to the intellectual and social world around him by demanding of himself to deal with the world rationally, in fact to deal with it to the utmost limits that rationality would allow. Already, when young, he honed his ability to ascertain the causal relationships that operated in situations that involved well defined ends. He also willed to limit personal pleasure in order to increase efficiency in securing ends. This rational approach worked well in the formal, narrowly circumscribed milieu of German academia that constituted his professional world until Hitler ejected him from it.

If self-containment and self-reliance mark his initial stance, his first insights into the meaning and the comforts of community came from his interactions with the circle of friends he acquired while in Gymnasium and his stay at the University in Frankfurt. Most of these friends came from emancipated Jewish families; their outlook was cosmopolitan, liberal, and they were open to the possibilities of the world. In this, too, he was following the footsteps of his father, who had married a woman with a similar cultural background. The full force of the warmth and the strength of that community struck Bethe when, in 1929, he became Paul Ewald's assistant in Stuttgart. Ewald -- whose wife was the niece of a famous and influential reform rabbi -- opened his home to the young Bethe and he became a frequent visitor. Ten years later he married one of Ella and Paul Ewald's daughters, Rose.

One can identify three fairly well delineated "stages" in Bethe's life until the mid-50's. In the period from 1906 till 1933, it is German culture and German institutions that mold him. The two Handbook der Physik articles are the fruition of stage one. From 1934 till 1940, Cornell is his haven. The "Bethe Bible" and his solution of the problem of energy generation in stars epitomize the capacities of the mature scientist, of the scientist who has helped shape the new field of subatomic phenomena. The third period, which begins with the outbreak of World War II, sees him acquiring new powers at the Radiation Laboratory at M.I.T. and at Los Alamos, and also gaining wisdom. The postwar years from 1946 to 1955 constitute one of the most exhilarating phases of his life, both scientifically and professionally. The stage of his activities has become national and international. He is at the center of important new developments in quantum electrodynamics and meson theory. He helps Cornell become one of the outstanding universities in the world. He is a much sought after and highly valued consultant to the private industries trying to develop atomic energy for peaceful purposes. He is deeply involved in, and exerts great influence on, matters of national security. He is happily married and is the proud father of two very bright children. But the demands from his activities outside Cornell were enormous, the pace grueling, and they were exacting a heavy toll both at home and in his scientific researches. In 1955 Bethe went to Cambridge University to spend a sabbatical year there. It was a year of taking stock and of narrowing the focus of his scientific researches.

In 1958, in a review of Robert Jungk's Brighter than a Thousand Suns, Bethe revealed some of his despondency.¹⁰ He there recalled the "golden times" of the 20's and 30's when science for him was "a great spiritual adventure." All experimental discoveries were made with small, rather inexpensive apparatus, every detail of which could be understood. One could then know all of physics. And most importantly, "The physicists in all countries knew each other well and were friends. And the life at the centers of the development of quantum theory, Copenhagen and Göttingen, was idyllic and leisurely, in spite of the enormous amount of work accomplished." He confessed that he could not help but be nostalgic reading about these times in the book. "How it has all changed! There are now enormous accelerators, with large groups of scientists working on each, a wealth of detailed material is published in highly specialized journals every week so that it has become impossible to keep up with the literature even in a narrow part of nuclear physics.... The life of physicists has changed completely, even of those not involved in politics or in technological projects like atomic energy. The pace is hectic. Yet the progress of fundamental discovery is no faster, and perhaps slower, than in the thirties."¹¹

There has been a gradual shift in the position where Bethe places the fulcrum in his attempt to resolve the tension between individualism and community, between dissension and solidarity. In a world where no one can master all knowledge and skills, communities assume new dimensions as dynamic, generative enterprises that produce new knowledge, new tools, new generations of practitioners, and that give coherence and meaning to the human enterprise.

For Bethe, the practice of science has been a "great spiritual quest," a search for understanding. Just as with his belief in a pre-established harmony there is a Leibnizian component to his interpretation of understanding. It is human understanding, but in understanding we partake in what Leibniz called divine understanding, and we fathom some of the rational and logical of the world. But precisely because we can only have human understanding, Bethe believes that all understanding -- and scientific understanding in particular -- requires that one test and risk one's convictions and prejudgments in and through encounters with others, and demands confronting what is new and alien. The scientific community provides the channels and the setting for such encounters. It is a community that relies on the virtues of honesty, tolerance, trust, truthfulness, and cooperation; one that guarantees tolerance for the views of others and the verification of their claims. It is the community that authenticates the integrity of our scientific understanding. Without a moral commitment on the part of scientists to be truthful and trustworthy members of this community, no important statements of fact could see the light of day and be stabilized.¹²

As he grew older Bethe valued ever more his membership in the various communities that allowed him to be productive, to continue growing, to keep on constantly becoming. Still, it is one of Bethe's striking characteristics that there is, so to say, only one of him. He is the same whether dealing with a student, with a colleague, with the president of Cornell, or with a senator in Washington. It is an expression of the integrated nature of his self. It is an expression of his wholeness, of his integrity. But, of course, no person is whole; every person is in some ways fractured; every person is the reconstituted entity of the fragmented pieces of her past.

Yet Bethe's striving for unity has been constant throughout his life. He has constantly striven to meld his "self" and his "self-representation":¹³ they are an amalgam of what was and is important -- psychologically, intellectually, and culturally -- in his continuous and continuing development. In a deep and interesting way he exemplifies the Western notion of the autonomous self as culturally bound. But as Geertz reminds us:

"The Western conception of the person as a bounded, unique, more or less integrated motivational and cognitive universe, a dynamic center of awareness, emotion, judgement, and action organized into a distinctive whole and set contrastively both against other such wholes and against its social and natural background, is, however incorrigible it may seem to us, a rather peculiar idea within the context of the world's cultures."¹⁴

The Biographical Task

Over the past four years I have been diligently collecting and studying all the materials historians amass when writing biography. To date Bethe has published some three hundred scientific and technical papers. Many of these are quite lengthy review articles, of the same quality and comprehensiveness as the early Handbuch articles and the "Bethe Bible" on nuclear physics. The latest is a fifty page article on the theory of Type II Supernovas that appeared in the Review of Modern Physics two years ago. He has also authored a very large number of reports and papers that were originally classified. Some of these have become classics upon declassification even though they have never been published in the open literature. To give an indication of the magnitude of the corpus of these works, in the summer of 1991 I obtained from the Los Alamos Archives a five hundred page report on shock waves that Bethe had written during the war and that I had requested to be declassified. It contains much that was original and new at the time that it was drafted. Bethe has also written extensively on the issues connected with weapons proliferation, test ban treaties, ABM, disarmament, nuclear power, and SDI. There are by now some forty cartons of Bethe papers in the Cornell archives. They contain only materials up to the early 80's: correspondence, drafts of published and unpublished papers, calculations, lecture notes, class notes, and his extensive notes on the numerous seminars and conferences that he attended. Also in the Cornell archives are the records connected with his role as advisor to various government agencies, national laboratories (Los Alamos, Oak Ridge, Brookhaven) and those connected with his extensive consulting activities with industrial laboratories, such as G.E., Detroit Edison and Avco.

The vast correspondence between Bethe and Peierls, Sommerfeld, Weisskopf, Rabi, Oppenheimer, Feynman are on deposit in various archives. His former students have kept the letters they received from him; the record of their interactions often fills many notebooks. And there are over seventy graduate students who did dissertations under Bethe's guidance! There is also an extensive extant correspondence between Bethe and his mother, and since 1968 he has kept a detailed diary. One is thus overwhelmed by documents, letters, and papers. It is clearly impossible for me to master this wealth of information in the finite amount of time available. Yet until last summer I believed that my biography ought to be as detailed as the available materials allow. But last fall, as I surveyed what I had already written and what had yet to be written, it became apparent that the biography thus conceived would take up three volumes that no one would read. This would be unfair to Bethe (for I believe his life ought to be read), and would also be unfair to me. My aim at present is to write a book of some five hundred pages in length, that would be accessible to a wide audience. Although it would include a discussion of Bethe's scientific output, I am planning to edit a separate volume of Bethe's most important scientific papers, a volume that would contain extensive comments to set them in context.

The challenge in writing the biography has therefore become selection.¹⁵ To give a more balanced presentation I have decided to write a good deal of the biography as studies in parallel lives. There will be separate chapters on Bethe and Peierls, Bethe and Gamow, Bethe and Teller, Bethe and Weisskopf, and Bethe and Oppenheimer. Constantly comparing Bethe with some of his contemporaries with whom he interacted strongly, prevents hagiography. The approach illuminates Bethe's strengths but also makes clear his limitations. It highlights the roles culture and institutions play in shaping metaphysical outlooks, building confidence, and molding character. It also emphasizes the role of contingency in transforming lives. Before turning to the high points of the chapter on Bethe and Gamow let me illustrate this last point with an observation stemming from the study of the parallel lives of Bethe and Peierls.

Rudolf Peierls is one year younger than Bethe. He grew up in Berlin in a somewhat similar familial and social background. Both of Peierls' parents were assimilated German Jews; they had, in fact, converted to Christianity when the young Peierls was five years old. Bethe first met Peierls in Sommerfeld's seminar when Peierls came there in 1927. They became close friends. Yet interestingly, in their extensive correspondence until the early 30's they addressed one another as "Sie" rather than the more informal "du." Both had to emigrate from Germany in 1933 and both went to England. Peierls remained in England when Bethe accepted his visiting appointment at Cornell in 1935. As is well known, Peierls played a crucial role in establishing the feasibility of an atomic bomb; Bethe and Peierls were together at Los Alamos, Peierls being a group leader in Bethe's Theoretical Division, in charge of the implosion calculations. Peierls returned to England in 1946, undoubtedly the leading British expert on atomic energy and nuclear weapons. Yet the British political system is such that Peierls was never consulted on any of these matters. The British government relies on the staff of Harwell, the British Atomic Energy laboratory, and other governmental agencies for advice relating to nuclear issues, whether they be technical such as the building of an H-bomb or they be economic or political.

There is little question that Bethe would have stayed in Great Britain had he been offered a job there in 1934. Would he have remained there, it is clear that his life would have been very different. The scope of his political activities after World War II would surely have been narrower and his potentialities in that sphere would probably never have been fully realized. And I would argue that Bethe's activities as a citizen and a statesman after World War II were more important than his role as a productive scientist.

Bethe and Gamow

George Gamow presents an interesting contrast to Bethe, and their intellectual biographies constitute a fascinating study in parallel lives. During the 30's their interests overlapped. Gamow's explanation of α-particle decay in 1928 was the first application of wave mechanics to nuclear physics; it suggested that this domain might also be understood in terms of the new quantum mechanics. In 1930, he wrote the first monograph on nuclear physics that attempted to account for the properties of nuclei using quantum mechanics.¹⁶ Starting in 1928 Gamow began applying the lessons being learned in nuclear physics to astrophysical problems. After the discovery of the neutron and, subsequently, of artificial radioactivity, he was one of the first to address the problem of nucleosynthesis in stars. A man of wide interests and great imagination, he seems to have been more interested in addressing "big questions" and offering plausible answers than in carefully working out the details of his suggested answers. The big questions ranged from the birth and death of the sun, the genesis of the chemical elements, the origin of the planetary system, to cosmogenesis and the alphabet of the genetic code. In 1937, he was ideally positioned to solve the problem of energy production in stars. He recognized the interrelation of nucleosynthesis and energy production. Together with Edward Teller, his colleague at George Washington University, he fashioned the tools to solve the problem. Perhaps because of his fascination with problems of origins and genesis, he came to regard nucleosynthesis as the all-important problem and the explanation of the relative abundances of the elements the criterion by which the theory would be tested. He was unable to see that energy generation and nucleosynthesis need not be addressed simultaneously. Bethe -- always the theoretician who based himself on firm empirical data and sound phenomenological knowledge -- decoupled the two aspects of the problem and was thus able to give the definitive answer to the problem of the energy generation in stars. One other characteristic difference between Bethe and Gamow is responsible for Bethe's success in solving the problem. Bethe is a calculator, a reliable calculator. No calculation fazes him, however difficult or involved it might be. He will sit at his desk, day in, day out, calculate analytically whatever can be calculated exactly, or make the necessary justifiable approximations to obtain the sought-for answer. And the answer will always be converted into numbers that can be compared with empirical data. Gamow on the other hand had a fertile imagination, but his calculations were always suspect. He turned to his friends, Edward Teller and John von Neumann, for help with the mathematics and to his students for carrying out computations. Teller did most of the mathematics and calculations in their collaborative work during the thirties.

George Gamow was born in Odessa on March 4, 1904.¹⁷ His father was a teacher of Russian literature at one of Odessa's private lycées for boys.¹⁸ His mother's family stemmed from a long line of Ukrainian clergymen. She died when Gamow was nine. As a boy, Gamow was precocious and talented in many different areas. At age seven his mother was reading him Jules Verne and he dreamt of going to the moon. He went to high school during the 1917 revolution and the subsequent Civil War. For a while, after he had become fascinated by number theory, topology and set theory, he thought of becoming a mathematician. But physics won out. He enrolled in the physico-mathematical faculty of Novorossia University of Odessa in 1922 but left after a year to go to Petrograd. Petrograd was the scientific capital of the USSR; the Academy of Sciences and its main academic institutes were located there. The physical sciences, and physics in particular, were being nurtured in Petrograd and had recovered from the upheaval of the revolution. Petrograd had two higher educational institutions, the Polytechnical Institute and the university, and both offered a sound training in physics. Gamow enrolled at the university. He there came under the influence of Alexander Alexandrovich Friedmann.¹⁹ Friedmann was trained as a mathematician and was deeply interested in physical problems. In 1922, after studying general relativity, he discovered a mistake in Einstein's treatment of the cosmological problem and indicated that the equations of general relativity admit non-static solutions corresponding to both open and closed universes. In 1923-24 Friedmann taught a course at the university on the mathematical foundations of Einstein's theory of general relativity which Gamow attended. This was Gamow's introduction to both general relativity and to the concept of an expanding universe. Fellow students in the class were Lev (Dau) Landau and Dmitry (Dim) Ivanenko, and the three of them became inseparable friends. This brilliant trio became known as the three musketeers. Ghenia Kanegisser, who later became Ronald Peierls' wife, was another member of this social circle. There were no senior theoretical physicists in there at the time, and since the USSR had abolished formal higher degrees, the three musketeers were left to their own devices. They taught themselves physics, and mastered the new quantum mechanics. The lack of mentors in both the intellectual and the political realm had important consequences in the subsequent careers of Gamow, Ivanenko and Landau. The discipline that comes from a rigorous graduate education was not a pronounced feature among Gamow's traits.

Although the first half of the 1920s was also a period of great political and economic instability in Weimar Germany, the German universities escaped some of the turmoil, and were able to maintain a semblance of stability. Göttingen and Munich, in particular, were able to carry on more or less as before the war, but under much more stringent economic conditions.

Through Sommerfeld -- whose seminar in Munich he entered in 1926 -- Bethe was introduced to the Göttingen view of the pre-established harmony between physics and mathematics. The beauty and utility of complex variables, and the unification of geometry and gravitation implicit in Einstein's general theory of relativity had deeply impressed Sommerfeld, and he required all his Doktorants to master, in addition to the traditional sub disciplines of theoretical physics, most branches of analysis. Sommerfeld's powers were such that he had a command of all of physics and most of mathematics until the early nineteen thirties, and his erudition certainly impressed Bethe. But more important for the young Bethe was the appreciation that Sommerfeld's mastery of all of physics and most of mathematics was his anchor in integrity.

Sommerfeld was a forceful and charismatic figure, and though he was very much the Herr Geheimrat, nonetheless the atmosphere of the seminar was characterized by the intellectual give-and-take between him and his students and assistants. Among them are to be found some of the outstanding theorists of their generation: Peter Debye, Saul Epstein, Paul Ewald, Max von Laue, Wolfgang Pauli, Werner Heisenberg, Gregor Wentzel, Fritz London, Albrecht Unsöld and Bethe.²⁰ All were superb theorists and all were powerful calculators. It is a measure of the man that Sommerfeld never felt threatened by the brilliant students he trained. Sommerfeld learned from his students and assistants and collaborated with them. After they left the seminar they wrote to him and kept him informed of the latest developments. His students fashioned new tools that he, the master craftsman, learned to use. He thus continued to grow and to adapt to the new topography of theoretical physics. After the advent of quantum mechanics he was one of the first to integrate the new materials into a textbook.

It was in Munich that Bethe anchored his self-confidence. He there discovered his exceptional talents and his extraordinary proficiency in physics. Sommerfeld gave him indications that he was among the very best students who had studied with him. This self-confidence in matters of physics quickly extended to other matters.²¹ His enormous self-confidence also gave him the courage to face the world in moral terms.

Sommerfeld had a deep commitment to the school of theoretical physics he established in Munich. He had several opportunities to leave Munich for more prestigious positions,²² yet he remained in Munich. Bethe learned from Sommerfeld what commitment to an institution and to a tradition meant. Bethe would emulate his teacher's comportment and at Cornell he too built a school of physics where he trained and influenced some of the outstanding theoretical physicists of their generation: Konopinski, Marshak, Feynman, Dyson, Dalitz, Salpeter, Goldstone, Thouless, Carruthers, Jackiw, Negele. After World War II, Bethe received many lucrative professorial offers, including a request that he succeed Sommerfeld in Munich. He turned them all down, including an invitation from Oppenheimer in 1953 to join the Institute for Advanced Study.²³

The other great formative influence on Bethe was Fermi. Fermi helped Bethe free himself from the rigorous and exhaustive approach that was the hallmark of Sommerfeld. While on a Rockefeller Fellowship in Rome in 1930, and again in 1931, Bethe learned from Fermi to reason qualitatively, to obtain insights from back-of-envelopes calculations, to think of physics as easy and fun, as challenging problems to be solved; he discovered what "lightness" meant. In Rome he also was exposed to the much freer and more informal mode of interaction between Fermi and his students than had been the case in Munich.

Bethe's craftsmanship is an amalgam of what he learned from these two great physicists and teachers, combining the best of both: the thoroughness and rigor of Sommerfeld with the clarity and simplicity of Fermi. This craftsmanship is displayed in full force in the many "reviews" that Bethe has written. His first was the result of Sommerfeld asking him to collaborate in the writing of his Handbuch der Physik entry on solid state physics. This was the first of many reviews, of which the two Handbuch entries and the "Bethe Bible" on nuclear physics are merely the most famous. I call them reviews, but this is a misnomer. They are really syntheses of the field under review, giving them coherence and unity, charting the paths to be taken in addressing new problems. They usually contain much that is new, materials that Bethe worked out in the preparation of the essay.

Before World War II Bethe derived a sense of unity from his mastery of all of physics, and in particular the newer fields of physics that were being probed by the new quantum mechanics and by the new instrumentation in the nuclear realm, such as cloud chambers, cyclotrons and β-ray spectrographs. For the most part, quantum mechanics could successfully account for the structures that were observed in the physical realm of nature: nuclei, atoms, molecules, solids, stars. Bethe contributed importantly to each of these fields, often shaping the newly emerging sub disciplines. But it should be stressed that he never attempted to "unify" these fields in the sense of trying to find a unitary theory that would encompass all the phenomena in all these realms.

Energy Generation in Stars

Gamow's outstanding talents were recognized by Abram Ioffe and in 1928 he was sent to study with Bohr at his Theoretical Physics Institute of the University of Copenhagen. The fellowship gave him the opportunity to visit the centers of theoretical physics in Germany and England. Leon Rosenfeld, who became a distinguished nuclear physicist and was a close associate of Niels Bohr, recalled that his experience with nuclear physics started in the summer of 1928 with the sudden appearance one morning of Gamow in the library of the physics institute at Göttingen. Rosenfeld described Gamow as "a fair-headed giant with shortsighted, half -- shut eyes behind his spectacles."²⁴ Gamow explained to Rosenfeld that he was working on the quantum mechanical description of α-radioactivity in heavy nuclei. This at a time when essentially all that was known about nuclei was that they were very small, had an electric charge, and had a spin. It had become clear to Gamow that one must take into account the fact that the α-particle was quasi bound inside the nucleus before it came out. Thus the α-particle must experience an attractive potential inside the nucleus, which combined with the repulsive potential outside to form a "potential barrier" through which it could "tunnel." The decay of radioactive elements is a purely quantum mechanical process in which the α-particle "leaks through" the nuclear potential. Gamow constructed a solution of the Schrödinger equation for this type of potential, which satisfied the boundary condition that there was an outgoing wave at large distances (corresponding to the escaping α-particle). The quantum mechanical formula he obtained for the "transparency" of the nuclear walls allowed him to derive the Geiger-Nuttall relation between the lifetime of the nucleus and the energy of the emitted α-particle. Gamow's calculation was the first application of quantum mechanics to nuclear physics. (Edward U. Condon and Ronald W. Gurney gave the same explanation at the same time).

At Göttingen that summer Gamow met Fritz Houtermans.²⁵ Houtermans and Gamow were kindred spirits -- cosmopolitan, bons vivants, somewhat reckless, the carriers of a vast store of jokes they loved to tell, and displaying at times curious manners. They became very good friends. Hendrik Casimir noted in his autobiography that Houtermans "was so colorful a person that one is liable to forget that he was also a very good physicist." Houtermans had obtained his Ph.D. working with James Frank on resonance fluorescence in mercury. After he heard Gamow expound his theory of nuclear α-decay he discussed with Gamow refinements of the theory and its application to specific nuclei. A joint paper ensued which was submitted for publication to the Zeitschrift für Physik in October 1928, by which time Gamow had gone to Copenhagen to visit Bohr and his Institute, and Houtermans had moved on to Berlin. Houtermans not only had fully mastered Gamow's theoretical description of α-decay but he also realized that the quantum mechanical tunneling effect made possible the inverse process in which a nucleus absorbs an α-particle and subsequently gives rise to various reaction products. All nuclei are positively charged and in order to react they must penetrate their mutual Coulomb barrier. Gamow's theory indicated that in general the effective cross-section for nuclear reactions is given essentially by the product of: 1) the collision cross-section (Houtermans took it to be equal to πr₀² where r₀ is the nuclear radius); 2) the transparency of the Coulomb barrier for the incident particle; and 3) the probability of a disintegration when the nuclei are together. On the assumption that the latter is essentially 1, Houtermans²⁶ determined that the cross-sections for such reactions were too small to be easily detected experimentally at the kilovolt energy range, the laboratory energies then available.²⁷

In discussions with Robert d'E. Atkinson, a young English astronomer who had attended Eddington's lectures at Cambridge on the internal constitution of stars, and who was visiting Göttingen in the summer of 1928, it became clear that stellar interiors were hot enough for sustained nuclear reactions initiated by protons to take place. Atkinson and Houtermans realized that in the calculation of the transparency of the nuclear barriers for the thermal protons in the interior of a star one should not take for the proton energy the mean energy 1½ kT, since according to the Maxwellian distribution there are always present fast protons in small numbers and these are very much more efficient in their disintegration effect. With some help by Gamow, Atkinson and Houtermans wrote a joint paper on "thermonuclear" reactions in which they obtained an expression for the dependence of the reaction rate on the temperature of the mixture and on the atomic numbers of the colliding nuclei. In their paper Atkinson and Houtermans gave a schematic representation of the number of particles N(E) and of the cross-section for disintegration σ(E) as a function of the energy of the particles, in order to highlight the fact that the total number of disintegrations, which is essentially given by the product N(E)σ(E), reaches a maximum for an energy that is well above the average energy corresponding to a given temperature.²⁸ They then computed the probability per unit time of nuclear reactions between nuclei of charge Z and mass A and a proton gas with a density of 10²³ particles/cm³ and Maxwellian velocity distribution corresponding to a temperature of 6 x 10⁷°K. They estimated the periods of time necessary for 50% transformation of different substances mixed up with hydrogen under conditions holding inside the sun, on the assumption that each penetration gives rise to disintegration.²⁹ Their calculations indicated that for heavy elements the probability of processes of this type were exceedingly small, but for light elements (assuming a collision radius of 4 x 10^-13 cm) they obtained half-lives ranging from 8 seconds in ⁴He, to 34 minutes for Li, 14 years for B, and 10⁹ years for ^2ONe and 10⁶¹ years for Pb.³⁰

Writing at a time when the only information available was based on α-particle bombardment -- no accelerator existed and thus no experimental data on proton reactions was available -- they concluded that the only reactions suitable for explaining energy production in the sun are those between protons and the nuclei of some light elements between lithium and neon in the periodic table. They suggested that the process was a cyclic one in which four protons are captured consecutively by a nucleus and then ejected in the form of an α-particle. The tentative title of the paper Atkinson and Houtermans submitted to the Zeitschrift für Physik was "Wie kann man ein Helium Kern in einem potential Topf kochen?" ("How can one cook a helium nucleus in a potential pot?")³¹ Houtermans later recalled that

"that evening, after we finished our essay, I went for a walk with a pretty girl. As soon as it grew dark the stars came out, one after another, in all their splendor. "Don't they shine beautifully?" cried my companion. But I simply stuck out my chest and said proudly: "I've known since yesterday why it is that they shine."

Later that year, after his discussions with Houtermans on nuclear reactions, Gamow visited the Cavendish Laboratory and expounded his findings. He stressed the fact that from his theory the transparency of nuclear barriers is much larger if the bombarding particles are protons. Due to the smaller charge and mass of a proton the probability of penetration of a proton into a given nucleus is the same as of an α-particle with 16 times larger energy. Since Rutherford had observed nuclear transformations only for α-particles of several million volts one could conclude that in the case of proton bombardment one could expect observable results already for energies of a few hundred kilovolts. These considerations played a crucial role in the decision to have Cockcroft and Walton build at the Cavendish a high voltage apparatus that could produce a beam of 500 keV protons. Bombarding a target of lithium with these protons, Cockcroft was able to detect the reaction

p + ₃Li⁷ → ₂He⁴ +₂He⁴

with the liberation of 18 Mev, a large amount of energy. The discovery of the neutron in 1932 by Chadwick, and the subsequent systematic analysis of neutron-induced nuclear reactions by Fermi initiated a new stage in the study of the "thermal transformations of elements in stars." Henceforth the subject matter of nucleosynthesis could be addressed empirically. Gamow became fascinated by the subject. At a symposium on "The Nucleus of the Atom and its Structure" in the 1935 he spoke on "Nuclear transformation and the origin of the chemical elements" and in his lecture he noted that³²

"there are many different phenomena inside a star giving rise to formation and transformation of different elements and the hope may be justified that further investigation will clarify the relative importance of various processes and lead to a complete explanation of the relative abundance of different elements in the universe."

Nor was Gamow alone in his interest in this problem. In the fall of 1936, Rutherford delivered the Henry Sidgwick Memorial Lecture³³ in which he reviewed the developments in the physicist's understanding of the structure of nuclei since the beginning of the century and described the important new findings since the discovery of the neutron by Chadwick in 1932. He indicated that during "the last few years ... almost all elements had been shown to be capable of transformation by suitable agency," in particular by bombardment by fast neutrons, and concluded his lectures with the observation that:³⁴

"The information we have gained on transformation processes may prove of great service too in another direction. In the interior of a hot star like our sun, where the temperature is very high, it is clear that the protons, neutrons, and other light particles must have thermal velocities sufficiently high to produce transformation in the material of the sun. Under this unceasing bombardment, there must be a continual process of building up new atoms and of disintegrating others, and a stage at any rate of temporary equilibrium would soon be reached. From a knowledge of the abundance of our earth, we are able to form a good idea of the average constitution of the sun at the time 3000 million years ago when the earth separated from the sun. When our knowledge of transformations is more advanced, we may be able to understand the reason of the relative abundance of different elements in our earth.... We thus see how the progress of modern alchemy will not only add greatly to our knowledge of the elements, but also of their relative abundance in our universe."

What Rutherford had in mind was not merely the relative abundance of the elements, but also the relative abundance of the isotopes of a given element -- that is, of nuclei with the same number of protons but differing number of neutrons.³⁵ Isotopes behave alike in the chemical and the physical processes responsible for geological change. Only nuclear processes differentiate isotopic differences. Hence isotopes reveal the nuclear history of the cosmos.

In 1936, while writing the second edition of his monograph on nuclear physics, Gamow reviewed the data on the relative abundances of the elements that Aston and others had obtained and was struck by "the remarkable constancy in isotopic constitution of the elements found on the earth [in the universe]." It was generally believed that the data represented a "universal" or "cosmic" abundance. For Gamow, explaining theoretically this uniformity of the relative abundance of the different nuclei in the universe became the interesting puzzle, and his investigations of "the creation and transformation of elements in different parts of the universe" had as their objective the finding of a solution to that problem. Assuming that protons, neutrons and electrons were the fundamental building blocks, he suggested that "by some process taking place inside a star ... a considerable amount of neutrons can be produced." When these neutrons were slowed down they would be captured by nuclei, especially heavy nuclei, in reactions such as

_ZX^A + ₀n¹ → _ZX^A+l + hυ (1)

forming heavier isotopes, which by the process of β-decay

_ZX^A+1 → _Z+1X^A+1 + e- + neutrino (2)

may give rise to elements located higher in the periodic table. He believed that such reactions might be effective throughout the whole range of elements and could lead to an understanding of the relative amounts of the various elements in the universe. He concluded his section with the statement:³⁶

"We may also notice at this point that reaction-chains of the type (1) (2), corresponding to the building up of heavy nuclei, must in general involve the emission of large amounts of energy -- probably enough, in fact to provide a source for the radiation of the stars, which for a long time presented an inexplicable puzzle to astrophysicists."

Explaining energy production was an addendum to explaining nucleosynthesis. In preparation for the forthcoming fourth Washington Conference on Theoretical Physics, Gamow in the fall of 1937 asked his graduate student Charles Critchfield to calculate the cross-section for the reaction

p + p → d + e⁺ + υ (3)

under stellar conditions in order to see whether the cyclic reactions that von Weiszäcker had proposed -- in which reaction (3) is the first step in the conversion of four protons into an α-particle -- could account for the luminosity of the sun. But there was another reason for Gamow to investigate the reaction which he divulged in the lectures he delivered at the Institut Henri Poincaré‚ during the summer of 1938:³⁷

"The energy liberated [in the carbon cycle that Bethe had put forward after the Washington conference] suffices to explain the radiation from stars, but it does not produce free neutrons. If we want to explain the formation of the heavy elements, we must envisage another reaction that produces neutrons in great quantity."

That is why reaction (3) was so important to Gamow. Once deuterons are produced they react to copiously produce neutrons:

d + d → ₂He³ + n

which neutrons in turn help generate the heavy elements through neutron capture reactions.

Bethe helped Crichfield with the calculation of the pp→de⁺υ reaction, and was well prepared to absorb what the astrophysicists, in particular Stromberg and Chandrasekhar, told the nuclear physicists at the Washington Conference.

Following the Washington conference Chandraskhar, Gamow and Tuve issued a brief report on the meeting in Nature.³⁸ In it they reported that "some interesting conclusions had been reached" concerning nuclear transformations as the source of energy production in stars. They noted that the so-called "Aufbauhypothese" -- the suggestion that the production of the heavier elements from hydrogen is continually taking place in the interiors of stars and that in the process energy is liberated -- had run into difficulty. Recent experiments had indicated that ⁵Li and ⁵He were both unstable. Thus the reaction

¹H + ⁴He → ⁵Li → ⁵He + β⁺

could not be invoked, which in turn invalidated von Weizsäcker's proposal for the build up of the heavier elements. Similarly, the other possible chain of reactions for the synthesis of heavier elements from hydrogen would require the stability of ⁸Be, but recent experimental evidence indicated that this was not the case. Chandraskhar, Gamow and Tuve therefore concluded that

"the only course not excluded by present evidence on the binding energies of the light nuclei is the formation of ⁶Be in triple collisions involving an α-particle and two protons."

Although the existence of ⁶Be had not been established experimentally, they believed that it might be stable. They had estimated the probability of such reactions involving triple collisions and had concluded that "under the conditions existing in the interior of stars, the rate of energy production was sufficient to account for the radiation of the stars." They reported that as "another possibility" the reaction

¹H + ¹H → ²H + β⁺

was suggested.

"It seems that the rate of such a reaction under the conditions in stellar interiors would be enough to account for the radiation of the sun, though for stars much brighter than the sun other more effective sources of energy are required."

In addition, they also discussed the consequences of resonance effects in nuclear reactions for stellar models.

It is important to note that the Nature report indicates that in the minds of most of the participants the problem of nucleosynthesis was conflated with the problem of energy generation. This was certainly the case for Gamow and von Weizsäcker. Bethe separated the two problems. They would later once again be brought together, but at that stage the separation of the two problems was an important step for the resolution of both. When he got back to Cornell after the conference, Bethe investigated all the possible nuclear reactions that could be effective in energy production under stellar conditions, given the (then) empirically known properties of nuclei -- irrespective of how the reacting elements had been produced. He discovered the carbon cycle and computed the energy production that resulted from both the pp and the carbon cycle in stars. For these insights and calculations he obtained the Nobel Prize in 1967.

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